The
M3
introduced a
1130hp (834 kW) Sakae
21 engine and 20mm Type 99 Mk2 cannon with higher muzzle velocity and
100 rounds per gun. It first saw combat in the late spring of
1942, and Allied intelligence gave it the code name "Hamp" before
realizing it was an improved Zero. The improvements in speed,
maneuverability, and armament cost just enough range that the M3 could
not reach Guadalcanal from Rabaul.

The M5a first appeared
in August 1943 and was armed with two 20mm Type 99 Mk4 with belts of 85
rounds per gun. It could carry two 60kg (132 lb) bombs.

The 5c replaced both
cowling guns with 13.2mm
and optionally mounted another 13.2mm in the
fuselage. It also introduced pilot armor.

The A6M7 was build for kamikaze use, while the
A6M8c, with the 1560hp (1163 kW) Mitsubishi Kinsei 62
engine,
was not produced in quantity due to destruction of the Nakajima
factory.

The -K variants were two-seat advanced trainers. A total of 515
trainers were built by the Sasebo
naval arsenal (236 aircraft) and by Hitachi
(279 aircraft)

The legendary Zero was the terror
of the
Pacific for the first
year or so of the war. Accepted by the Navy in July 1940, it was just
coming into mass
production in
1941, and
the available Zeros (roughly 400 in number) were assigned to 1
Air
Fleet for the Pearl
Harbor raid and to
Tainan
Air Groupto cover the Philippines
invasion. Its range and low-speed maneuverability were
phenomenal, and it
was faster and had a better climb rate and service ceiling than most
first-generation Allied
fighters. Flown by superbly trained
pilots,
the
Zero quickly acquired an aura of invincibility in the eyes of Allied
airmen.

The aircraft was designed in response to a very
demanding set of requirements published by the Naval Aviation
Department in the spring of 1937. It needed the performance and
firepower to be an effective interceptor, the maneuverability to engage
enemy fighters, and the range to escort the G3M "Nell" on long missions. To meet
these conflicting requirements, the Mitsubishi design team, led by
Horikoshi Jirō, deliberately reduced the safety factors on many
structural components and made use of the new "Super Ultra Duralumin",
an aluminum-zinc alloy with remarkable strength.
Every effort was made to reduce drag, by using flush riveting,
retractable landing gear, a full canopy, and a "washout" wing tip
configuration.

Other notable features of the design included unusually
large
ailerons and an integral wing. The wing was a single unit, divided into
left and right fuel tanks whose large volume gave the aircraft its
remarkable range. The upper surface of the center of the wing was
the floor of the cockpit. This integral design contributed
structural strength and made the aircraft easy to assemble and
disassemble. Its low wing loading gave it the smallest turn radius of
any aircraft then in production, and its large ailerons gave it an
excellent roll rate. It was one of the first aircraft to be equipped
with a drop tank.

Although the Zero first saw combat over Chungking on 19 August 1940, production was initially slow. Some of the internal
opposition to the Pearl Harbor operation was based on the fear that
only 150 of the aircraft would be available by October 1941, which was
not enough for both Tainan Air Group
and 1 Air Fleet. One
bottleneck was the limited availability of 20mm cannon, and some of the
early training was conducted with
Zeros armed with only a single 20mm cannon.

Considerable information on the Zero was available to the Allies well before war broke out in the Pacific. Claire Chennault had studied the fighter in China, filed a report with Marshall in December 1940 that was passed on to Short at Oahu, and in July 1941 Chennault lectured a spellbound audience
of Hawaiian Air Force pilots for three hours on the flight
characteristics of the new Japanese fighter. Lieutenant Stephen Jurika,
an assistant naval attaché, actually managed to get into the cockpit of
a Zero on exhibition in Tokyo and later filed a detailed report with the Office of Naval Intelligence. However, the reports seem not to have been widely believed.

Disheartened survivors of combat with
Zeros reported that the aircraft engaged in elaborate acrobatics in the
middle of dogfights, as if taunting the Allied fliers.
These acrobatics may actually have been a means of communication or,
more likely, the unorthodox hineri-komi combat
maneuver taught to Imperial Navy pilots. Or they could have been
exactly what the Allied pilots thought they were. Bergerud quotes Jack Fletcher:

... at that time the Japanese Zeroes all wore Seven
League Boots and our aviators gave them a lot of ... respect.

However, the Zero had a number of
weaknesses that became evident
only after several months of combat. It was fragile, with
several
single-point failure points where one hit could destroy the entire
aircraft. It lacked self-sealing fuel tanks, making it a
firetrap.
As one American
flier observed, it was easily
destroyed once you had it in your sights; the trick was getting it into
your
sights.

Other weaknesses were discovered after the
Allies captured an
intact Zero in the Aleutians
in the summer
of 1942 and performed extensive flight testing. It was found
that the
Zero could not maintain a steep dive, and it lost maneuverability at
speeds
over 260 mph (418 km/h) or at altitudes above 15,000 feet (4600m). The
Zero’s
large propeller
generated high torque, so that its roll rate was much slower to the
right than
the left. Allied pilots were instructed that they
could shake a Zero off
their tail with a split-S to the right. Likewise, the
recommended attack
procedure was a diving attack followed by a sharp turn to the right.
The
U.S. Navy developed cooperative tactics such as the Thatch Weave that
took
advantage of the better radio communications gear carried by Allied
fighters. (The Zero was equipped with a very poor radio telephone, the
Type 96, that
was often
removed by its pilots to save weight.)

Horikoshi believed that the greatest flaw in his design
was its use of a relatively low-power aircraft engine, the 950hp (708
kW) Nakajima
Sakae. But this could not be helped. Japan's limited access to alloying
metals caused a shortage of the high-tensile steel used in more powerful
American aircraft engines, and in any case a larger engine would simply
have torn the fragile Zero apart.

The Zero’s armament was better
on paper than in practice.
Although the 20mm cannon shells packed a lot of punch, the guns had a
rather
low muzzle velocity, reducing their accuracy. They also had a
rather low
rate of fire. Perhaps this was just as well, given the
ammunition load of
just 60 rounds per gun. Bergerud quotes Sakai Saburo, the second
highest scoring Japanese ace to survive the war:

Our 20mm cannons were big, heavy and slow firing. It was
extremely hard to hit a moving target. Shooting down an enemy aircraft
was like hitting a dragonfly with a rifle! It was never easy to score
... our opponents were tough.

Another veteran, Fujita Iyozō, believed that an inexperienced pilot
could get just two bursts out the 20mm cannon before running out of
ammunition, while more experienced pilots could get five or six bursts.

The
0.303 machine guns in the engine
cowling were
often ineffective against sturdy Allied aircraft. The Zero had
an ammunition select switch that permitted its pilots to find the range
with
the 0.303s, then switching on the cannon to make the kill. However,
this was problematic, because the 0.303 rounds and the 20mm rounds had
very different ballistics. The Japanese
quickly became aware of the weakness of the original armament, and the
A6M3 used a more powerful 20mm cannon whose ballistics better matched
those of the 0.303 machine guns. Later models (beginning with late production versions of the A6M5)
increased the 20mm ammunition
loadout and replaced the light machine guns with more effective heavy
machine guns.

Operational units improvised a bomb rack for the Zero that allowed
it to carry a 250 kg bomb in place of the drop tank. This
fighter-bomber concept was taken up by the manufacturer as the A6M7,
which had a center bomb rack and fittings for two drop tanks outboard of the wing armament.

The Zero remained in production throughout
the war, and was
produced in greater numbers than any other Japanese fighter. However,
the
Zero was not a match for second-generation Allied fighters, such as the
Hellcat, in
spite of various design refinements. The replacement A7M Reppu
"Sam" suffered from the same flawed design philosophy as the Zero and
experienced repeated production delays that prevented it from ever
entering combat. By mid-1944 the Japanese were
experimenting with skip bombing using Zeroes armed with 250 kg (551 lb)
bombs, but the experiments were abandoned in favor of kamikaze attack.

The Zero required an overhaul every 150
hours of flight time, but
the Japanese often stretched this to 200 hours or more, with unhappy
results.

The official Allied code name for this
fighter was "Zeke," but Allied pilots usually referred to it as the
"Zero," which was the Japanese name for the aircraft (Reisen). However,
there was some tendency for Allied soldiers, sailors, and airmen to
call any
single-seat radial-engine Japanese monoplane aircraft a "Zero."